In a wireless communication system, a base station communicates with a plurality of remote terminals, such as cellular mobile telephones. Frequency Division Multiple Access (FDMA) and Time Division Multiple Access (TDMA) are the traditional multiple access schemes for delivering simultaneous services to a number of terminals. The basic idea underlying the FDMA and TDMA systems is based upon sharing the available resource, respectively as several frequencies or as several time intervals, in such a way that several terminals can operate simultaneously without causing interference.
In contrast to these schemes using frequency division or time division, Code Division Multiple Access (CDMA) allows multiple users to share a common frequency and a common time channel by using coded modulation. More precisely, as is well known to the person skilled in the art, a scrambling code is associated with each base station, and this makes it possible to distinguish one base station from another. Furthermore, an orthogonal code, known by the person skilled in the art as an OVSF Code, is allotted to each remote terminal (such as, for example, a cellular mobile telephone). All the OVSF codes are mutually orthogonal, thus making it possible to distinguish one remote terminal from another.
Before sending a signal over the transmission channel to a remote terminal, the signal has been scrambled and spread by the base section using the scrambling code of the base station and the OVSF code of the remote terminal. In CDMA systems, it is again possible to distinguish between those which use a distinct frequency for transmission and reception (CDMA-FDD system), and those which use a common frequency for transmission and reception but distinct time domains for transmission and reception (CDMA-TDD system).
Third-generation terminals, such as cellular mobile telephones, must be compatible with the UMTS standard, that is, they must be capable of operating under various wireless transmission standards. Thus, they will have to be capable of operating in a system of the FDMA/TDMA type, for example according to the GSM or GPRS transmission standard, or else in communication systems of the CDMA-FDD or CDMA-TDD type, for example by using the UTRA-FDD, UTRA-TDD or IS-95 transmission standards.
The invention thus applies in particular to all terminals or components of wireless communication systems, such as cellular mobile telephones for example, regardless of the transmission standard used. This is whether the latter provides for constant envelope modulation (GSM and DCS systems, for example) or variable envelope modulation (systems of the CDMA type), although the invention is especially advantageous with respect to variable envelope modulation systems.
The radio frequency transmission circuitry of a component of a wireless communication system, such as a cellular mobile telephone for example, comprises a power amplifier for amplifying the signal to a sufficient level for transmission. In systems operating according to the CDMA standard, which exhibits variable envelope modulation, use is made of linear transmission circuitry that makes it possible to resend the amplitude of the signal without distortion.
One approach for embodying the power amplification of the transmission circuitry includes using amplification circuitry of the delta-sigma type. An example of such an architecture is described, for example, in U.S. Pat. No. 5,777,512. Amplification circuitry of the delta-sigma type intrinsically exhibits the advantage of being more competitive in terms of efficiency than conventional linear amplification means. However, the use of amplification circuitry of the delta-sigma type exhibits drawbacks that will now be discussed.
Such amplification circuitry comprises frequency selector networks that make it possible to adjust the position of the zeros of the noise transfer function, that is, to adjust the frequencies at which the quantization noise is in theory eliminated. Also, traditionally, these zeros are placed in the useful or desired transmission band in which the signal is situated, so as to comply with the signal/noise ratio required by the transmission standard used.
Also, since the major part of the quantization noise is pushed out of the useful band of the signal, it is then necessary to provide one or more post-amplifier filters at an output for the amplification circuitry of the delta-sigma type. The function of the filters includes, in particular, eliminating the quantization noise outside the useful signal band. It is in fact necessary to comply with noise templates defined by the transmission standards, and for operation outside the useful band of the signal, the noise must not exceed a certain level of energy so as not to disturb other transmissions and receptions using different transmission standards.
To satisfy such a template, the filtering carried out by the post-amplifier filter is increased, and then there will be an inevitable increase in the losses in the useful signal band. It will then be necessary to put more power on the amplifier, thereby rendering it less competitive than a conventional amplifier.
Also, this problem of eliminating the noise outside the useful signal band is all the more complex to solve when the constraints imposed by the transmission standards, in terms of energy level, differ as a function of frequency. Thus, by way of example, if a mobile telephone is intended to operate according to the W-CDMA transmission standard, whose useful transmission band is situated between 1920 MHz and 1980 MHz, then the noise level outside the useful band should not exceed −117 dBm per hertz between 925 MHz and 935 MHz and −129 dBm between 935 MHz and 960 MHz so as not to disturb GSM receptions. Moreover, the noise level should not exceed −121 dBm per hertz between 1805 MHz and 1880 MHz so as not to disturb DCS receptions.